The permanent magnet type of stepping motor, with typically 200 steps per revolution, when used to drive a five-thread-per-inch screw on a machine tool, such as a milling machine, provides 0.001 inch deflection per motor step.
When finer resolution such as 0.0001 inch per step is required, (1) the pitch of the lead screw must be changed, or (2) gearing must be used between the motor and lead screw, or (3) a motor with finer steps must be used. All of these means reduce the maximum machine axis motion rate of the system by a factor of 10.
In the present system, a first 200 step/rev. type motor is used to drive the lead screw and a second cascaded motor, giving 2000 steps/rev., is used to rotate the first motor's housing. Through electronic circuitry, the 200 step/rev. motor takes all the full 1.8.degree. steps while the 2000 step/rev. motor makes up the remainder of the motion in 0.18.degree. increments. Thus, the motor system can be described as having 10 times the positioning resolution and 10 times the stepping rate of the 200 step/rev. motor alone while machine axis motion rate does not decrease. The second stepper is uniquely qualified to operate in the present system. It can easily provide steps as small as 0.18.degree. with exceptional drive stiffness (expressed in degrees of rotation per inch lb. of torque when backdriven). It has exceptional ability to handle the large inertia represented by the 200 step/rev. motor housing, both because of its large output torque and its drive stiffness.
The present system is particularly suitable where there is very little offsetting load once final position has been reached. If there is offsetting load, there will be a degradation of positioning accuracy as the axis is driven through the 200 step/rev. motor, whose stiffness is less than 1/10 as great as that of the second motor.
A system in which there is little or no offsetting torque when position is reached is one in which the load is pure inertia or where the hybrid system is used to drive a servo valve and hydraulic amplifier which in turn drives the machine axis.
In the present system, the second 2000 step/rev. motor mounts on a cylindrical coupling frame. The rear of the 200 step-rev. motor mounts on the second motor shaft via a motor shaft coupler. The front of the 200 step/rev. motor mounts on a coupling frame via a shaft bearing. The front of the coupling has a centering pilot and mounting holes. The 200 step/rev. motor drives the load. Assume that it is desired that the shaft rotate 180.72.degree.. This corresponds to 100 steps of the 200 step/rev. motor plus four steps of the 2000 step/rev. motor. The desired step count 1004 is shifted into electronic logic by which tens count and up, goes into a 200 step/rev. motor register and the units count into a 200 step/rev. motor register and the units count into a 2000 step/rev. motor register.
On the "go" signal, an oscillator feeds pulses to the 200 step/rev. motor driver and register, downcounting it, and also to the 2000 step/rev. motor driver and register, downcounting both simultaneously.
If it is desired to operate the 200 step/rev. motor faster than its start-stop asynchronous rate, (the rate at which the motor will start or stop without missing steps) the oscillator starts at a low rate and increases its rate according to an acceleration program. The rate starts to decrease when the count in the 200 step/rev. register has decreased to a predetermined amount so that the rate will be low enough to allow proper stepping of the 200 step/rev. motor when the register reaches zero. Since only a maximum of nine counts will at any time be in the 2000 step/rev. register, the associated motor can always be taken at the low rate. As they are stepped simultaneously with the 200 step/rev. motor, no additional operating time is caused by the added motor.
In the preferred embodiment of the invention, unidirectional position correction of the 200 step/rev. motor by the 2000 step/rev. motor is not possible. In other words, if the 2000 step/rev. motor were always to rotate in one direction, the limit of travel of the 200 step/rev. motor housing would be reached. Therefore, there are two rotational limit detectors on the 200 step/rev. motor housing. When a limit of rotation of the 200 step/rev. motor is reached as imposed by either of the limit detectors, the 2000 step/rev. motor corrects in the other direction. For instance, assume a desired motion of 180.72.degree.. Normally, as previously described, 100 steps of the 200 step/rev. motor and four steps of the 2000 step/rev. motor in the same direction for both motors will accomplish this. However, if the 200 step/rev. motor housing has rotated by the 2000 step/rev. motor to a predetermined position so as to reach a "stop" as imposed by one of the limit detectors, the limit detector causes the logic to put one more step in the 200 step/rev. count, overshooting by 1.8.degree. count, and the 2000 step/rev. motor is caused to operate in the opposite direction for six counts to arrive at the same position.
The system operates in this manner when either limit is reached.